use RewriteMap to redirect domains to subdomains

Question

I want to use RewriteMap to redirect domains to subdomains which in map.txt

RewriteMap domainMap txt://path/to/map.txt

map.txt contents :

exmple.com  mm.site.com
test.com    tt.site.com
domain.com  dd.site.com

I know following htaccess not correct but lt's near to what I want

Options +FollowSymLinks -MultiViews
RewriteEngine on
RewriteCond %{HTTP_HOST} ^(.*)\.site\.com$ [NC]
RewriteRule ^$ ${domainMap:%1} [L,R]

RewriteCond %{HTTP_HOST} !site\.com$ [NC]       
RewriteCond %{HTTP_HOST} ^(?:www\.)?(.+?)$          
RewriteCond %{REQUEST_FILENAME} !-f             
RewriteCond %{REQUEST_FILENAME} !-d         
RewriteRule (.*) /%1/$1 [L]